Today, we learn acoustics from yesterday's
equipment. The University of Houston's College of
Engineering presents this series about the machines
that make our civilization run, and the people
whose ingenuity created them.
Here's an old acoustics
book. It came out of England just after WW-II, and
it deals with a problem that'd been very much on
English minds -- locating enemy airplanes. Radar
had matured during the second world war, but it was
only starting to emerge from wartime secrecy. The
device everyone knew about was an odd, but much
simpler, listening machine. It consisted of a large
set of mechanical ears riding around on a
truck-drawn trailer.
The British sound locator was a brace of four
parabolic bowls, about 2-1/2 feet in diameter. They
worked best when they received sound at their
natural frequency -- 256 Hertz -- just under
middle-C. They were highly responsive at middle-C,
and increasingly directional at higher pitches.
The Germans, French, and Americans all had their
own versions of this technology. American sound
locators used arrays of trumpet-like ears -- kind
of like speakers on the old hand-cranked phonograph
machines.
But airplane speeds rose during WW-II and as they
did, sound locators became less and less effective.
They could hear an airplane 25 miles away. But they
were pure acoustical devices -- not electrical --
it took two minutes for sound to reach them. By the
end of the war, airplanes were staying close on the
heels of their own noise. Before the airplane noise
traveled 25 miles, the airplane itself traveled 16
or 20. By 1945, these acoustical ears gave hardly
any warning at all.
The sound mirror was another form of sound locator.
A sound mirror was a large partial-sphere with a
sensor at its center. It was usually an immovable
concrete structure, with a radius of curvature that
could reach 150 feet. It worked a lot like a
whispering gallery -- focusing sound from 15 miles
away onto a central point.
Of course, radar had already made dinosaurs out of
these beautiful old gadgets even before this
physics book came out. But But they represent a
wonderful lesson in practical acoustics. By the
time you've threaded through the equations and
diagrams, you know a great deal about sound
transmission.
A lot of yesterday's technology does that. If
you've ever worked on a steam engine, a tube radio,
or a Model-T transmission, you've seen their
workings spread out before you. Those old machines
taught you what no textbook could. Today's printed
circuits are far less willing to reveal the human
ingenuity that made them. And we're poorer because
of it.
I'm John Lienhard at the University of Houston,
where we're interested in the way inventive minds
work.
(Theme music)
